Method of and apparatus for digesting wood



June 16, 1931. P. E. HODGDON METHOD OF AND APPARATUS FOR DIGESTING WOODFiled May 27. 1929 Patented June 16, 1931 UNITED STATES PAUL E. HODGDON,0F MUNISING, MICHIGAN METHOD OF AND APPARATUS FOR-DIGESTING WOODApplication filed May 27, 1929.

My invention relates to paper pulp production and particularly toimprovements in the direct, or Ritter- Kellner process as used in themanufacture of sulphite pulp.

As is well known, the purpose of the cooking or digesting process is todissolve the lignin, sugars and resins of the wood, thereby separatingthe cellulose from the cementing materials with as little damage aspossible to the cellulose. The cellulose constitutes the useful elementfor paper making. Among the numerous processes heretofore suggested manyare objectionable either as to quality of pulp or the time and cost ofproduction.

The temperature and pressure maintained in the digester has a directrelation to the time required for the operation, temperature being themost important. The poorer quality of pulp is commonly cooked in from 8to 12 hours by the use of a high temperature. The pulp produced underthese operating conditions is not of high grade due to the physical andchemical conditions under which the material is treated. The wood issupplied to the digesters in the form of chips are insufficientlytreated.

After the chips have been charged into the digester, a period of fromtwo to three hours is usually required for heating the mixture of acidand chips and it has been found that during this period the temperaturemust not exceed 110 C. to prevent the chips from becoming case-hardened;that is, where the chips are subjected to a temperature in excess of 110C. an insoluble lignin compound is formed which renders such chips inertto the acid action, regardless of the length of time hr temperatureemployed in subsequent cooking. Where a digester is filled with coldchips and an attempt is made to raise the temperature quickly, there isdanger of local over-heating and burning at the steam inlet. I haveconceived the thought that sulphite pulp may be produced by a continuousprocess using preferably four containers, three of which would be inmany particulars, identical with digesters now in use and the fourthSerial No. 366,215.

in the form of a penetrator or pre-saturator into which the chips wouldbe charged.

Inasmuch as the speed of production of pulp is dependent to a largeextent upon the extent of penetration of the acid intothechips, and asthorough agitation is an essential to effective penetration, I haveprovided for mechanical circulation of the contents of the presaturatorpreferably employing indirect heat if necessary to supplement the heatsupplied by hot acid and by discharging the relief gases from thedigesters into the presaturator. I can, therefore, obtain effectiveresults in the penetrator by employing an acid having a low free S0 Thisis possible because more time can be taken in the presaturator forpenetrating than is practical to take in a commercial digester. Thetemperature can accurately be controlled at the point most desirable forpenetration. By thoroughly agitating and circulating the contents of thepresaturator, I am ableto effect the proper preparation of the chips forthe cooking process in the digesters. After the chips have conthoroughly penetrated, the contents of the Vessel may be pumped or blownintothe first digester where a proper cooking temperature isquickly'obtained due to the fact that the material is received alreadypenetrated and at a temperature of approximately 100 C. A shorttreatment in the first digester under ideal cooking temperatures, whichcan be more rapid than under the present conditions owing to the factthat the chips are already penetrated, is followed by the removal of thecontents through a disintegrating pump into the second digester wherethe cooking is continued with a further rise'in temperature. From thisdigester, the materials are similarly discharged into a third digesterwhere cooking is finished at a still further increase in temperature.Thus with approximately 1 hours in each of the four vessels, I am ableto produce a highly satisfactory pulp, the total time for thecombination of the processes being approximately six hours. This willalso be accomplished with a much lower maximum temperature than is usedat present which will insure a much stronger and better product.

' ing in which the figure is a side elevation of a series of tanks andpiping .connections arranged in accordance with my invention.

In the drawing, I have illustrated somewhat diagrammatically apresaturating tank 10 and digesters 11-12-13. A relief line 14 has avalved connection to each of the digesters and leads into thepresaturator.

A circulating system for the presaturator consisting of a pump 15, aninlet pipe 16 connected to the bottom of the tank at one end, and adischarge pipe 17 connected to discharge material into the opposite endof the tank. A coil 18 is indicated with the tank to indirectly heat thecontents if necessary. An acid inlet pipe is indicated at 19 by means ofwhich hot or cold acid may be supplied to the presaturator at the startof operations.

A pipe 20 leads from the presaturator through suitable valves to thefirst digester 11. A steam supply pipe 21 and an acid supply pipe 22 areshown and as each of the digesters is of duplicate construction, nofurther specific description thereof will be given. They may, however,be considered to be of standard form and having the usual linings.

After the chips have been cooked for a desired length of time in thefirst digester, the valves are manipulated and the disintegrating pump23 is started. The action of this pump which may be of the shippropeller or screw type, together with the pressure accumulated in thefirst digester will serve to quickly transfer the materialinto thesecond digester, where the cooking is continued with a gradual rise intemperature. Similarly, the materials will be transferred from thesecond digester into the third digester, by asimilar pump 24. In each ofthe digesters, the pressure is raised to the proper point by theadmission of hot acid as required and by utilizing steam to a limitedextent both for effecting circulation and for maintaining thetemperature. The admission of hot and new strong acid between thedifferent stageswill further hasten the cooking and insure a purerproduct.

Preferably the chips in the saturator will be'treated under idealconditions of acid and approximately 70 pounds pressure. The

transfer of the contents from the second digester to the third will bemade at a high temperature and the cooking will be continued in thethird and last digester at approximately pounds pressure. From the thirddigester, the ma'lerials will be discharged through the pipe 25 to theblowing pit's. Meanwhile, the other batches of material have beenreceived in the presaturator and passed on to the digesters. As alreadystated, if necessary the pressure in each digester can be the same bypumping new hot acid into the different digesters.

By the use of the process and apparatus described a substantial savingin heat is effected due to the reduction of time of treatment and to themore eflicient handlin of the materials. A further advantage is t at thepenetration may be carried on with a weak acid, because effected underlow temperature. For each condition of pressure or temperature in a diester, there can be only one concentration 0 gas, or in other words,sulphurous or free acid. Consequently if the penetration is eflectedbefore the chips are received in the digester, the same degree ofpenetration can be obtained with a weak acid as could be obtained with astrong one in the digester. Furthermore, by carryin on the preliminarysteps of penetration under ideal conditions, there is a higher yield ofcellulose due to the absence of local overheating of the chips aroundthe steam inlet such as occurs when the penetration takes place in thedigester.

The use of mechanical devices such as the disintegrating pumps that acton the chips in their passage from one to another of the digestersserves to accelerate the breaking down of the wood structure and toexpose greater areas of the chips to the action of the solvent. It willbe understood that the cellulose is not inert to the acid but merelymore resistant than the lignin and other constituents. sequently ifsubjected to the action of the reagent under extreme conditions ofmolecular activity incident to high heat for an extended period, thecellulose will be attacked and, if not carried into solution, will below in yield of the desirable beta and gamma fibers. Thus, bymechanically subdividing the wood pieces, the attack of the acid on thesoluble constituents is facilitated and this avoids the overcooking ofparts of the fiber during the effort of the acid to reach the innerparts of the wood chips. A high-er yield of pulp having better bleachingability and being lower in alpha cellulose will therefore result. Thoseadvantages together with a reduction of the cost of production renderthe described process particularly desirable.

I claim:

1. The method of digesting wood which consists in charging the woodchips into a presaturating tank, supplying acid to the tank Conconsistsin subjecting the wood chips to the action of acid while maintaining thetemperagas from a digester maintaining said temture at approximately 105degrees Cent. by indirectly applied heat, then transferring the chipsand acid to a digester without material loss of heat and cooking thechips in the digester at a higher temperature.

3. The. method of digesting wood which consists in subjecting the woodchips to the action of acid while maintaining the temperature atapproximately 105 degrees Cent; by indirectly applied heat, thentransferring the chips and acidto a digester without material loss ofheat and cooking the chips in the digester at a higher temperature, thenremoving the chips and acid from the digester and charging them into asecond digester where cooking is continued at a different temperature.

4. The method of digesting wood chips which consists in charging thechips into a container supplying acid thereto, raising the temperatureof the contents of the container to approximately 105 degrees Cent. byindirectly applied heat and by utilizing relief gas from a digestermaintaining said temperature substantially constant for a periodsufiicient to effect penetration of the chips by the acid, thentransferring the chips and acid to a digester and cooking the chips at ahigher temperature.

5. The method of digesting wood chips which consists in charging thechips into a container supplying acid thereto, raising the temperatureof the contents of the container to approximately 105 degrees Cent. byindirectly applied heat and by utilizing relief gas from a digestermaintaining said temperature substantially constant for a periodsuflicient to effect penetration of the chips by the acid, thentransferring the chips and acid to a digester without material loss ofheat and cooking the chips at a higher temperatnre.

6. The method of digesting wood chips which consists in charging thechips into a container supplying acid thereto, raising the temperatureof the contents of the container to approximately 105 degrees Cent. byindirectly applied heat and by utilizing relief perature substantiallyconstant for a period suflicient to effect penetration of the chi s bytheacid, then transferring the chips an acid to a digester withoutmaterial loss of heat and cooking the chips at a higher temperature,then transferrin the contents of the first digester to a secon digesterand continuing cooking at a different temperature.

7. The method of digesting wood chips which consists in charging thechips into a container supplying acid thereto, raising the temperatureof the contents of the con tainer to approximately 105 degrees Cent. byindirectly applied heat and by utilizing relief gas from a digestermaintaining said temperature'substantially constant for a periodsuflicient to effect penetration of the chips by the acid, thentransferring the chips and acid to a di ester without material loss ofheat and coo ing the chips at a higher temperature, then transferringthe contents of the first digester to a second digester'and continuingcooking at a difi'erent temperature,

and mechanically disintegrating the chips' in the process oftransferring the same to the second digester.

8. The process of digesting wood, which consists in placing wood chipsand acid in a container, maintaining the temperature of the contents ofthe container at approximately 105 degrees Cent., mechanically agitatingthe contents by circulation, then transferring the contents to adigester and cooking the chips at a higher temperature.

9. The process of digesting wood, which consists in placing wood chipsand acid in a container, maintaining the temperature of the contents ofthe container at approximately 105 degrees Cent. for approximately oneand one-half hours, continuousl mechanically agitating the contents bycirculation, then transferring the contents to a digester and cookingthe chips at a higher temperature.

10. The process of digesting wood, which consists in placing wood chipsand acid in a container, maintaining the temperature of the contents ofthe container at approximately 105 degrees Cent. mechanically agitatingthe contents by circulation, then transferring the contents to adigester and cooking the chips at a higher temperature, thentransferring the contents of the digester to a second digester andcontinuing cooking at a difi'erent temperature.

11. The process of digesting wood, which consists in placing wood chipsand acid in a container, maintaining the temperature of the contents ofthe container at approximately 105 degrees Cent. for approximately oneand one-half hours, continuously agitating the contents .by circulation,then transferring the contents to a digester and cooking the chips at ahigher temperature, then removing the contents of the digester,agitating the same to assist in disintegrating the tem erature forapproximately one and onehal hours then transferring the contents of thedigester while mechanically a itating said contents, to a seconddigester or further treatment.

13. The process of digesting wood, which consists in placing wood chipsand acid in a container, maintaining the temperature of the contents ofthe container at approximately 105 degrees Cent. for approximately oneand v one-half hours, continuously mechanically agitating the contentsby circulation, then transferring the contents to a digester and cookingthe chips at a higher tem erature for approximately one and one-halhours then transferring the contents to a second digester for furthertreatment at a different temperature.

14. The process of digesting wood, which consists in placing wood chipsand acid in a container, maintaining the temperature of the contents ofthe container at approximately 105 degrees Cent. for approximately oneand one-half hours, continuously mechanically agitating the contents bycirculation, then transferring the contents to a digester and cookingthe chips at a higher temperature for approximately one and one-halfhours then transferring the contents to a second digester for furthertreatment for ap roximately one and one-half hours at a d1 erenttemperature.

15. The method of digesting wood in a series of steps in differentcontainers which consists in presaturating wood chips in a containerunder theoretically correct conditions of temperature and acid strength,then transferring the chips and acid to a digester for further treatmentat a different temperature.

16. The method of digestering wood in a series of steps in differentcontainers which consists in presaturating wood chips in a containerunder theoretically correct conditions of temperature and acid strengthwhile continuously mechanically agitating the contents of the container,then transferring the chips and acid to a digester for further treatmentat a different temperature.

17. The method of digesting wood in a series of steps in differentcontainers which consists in presaturating wood chips in a containerunder theoretically correct conditions of temperature and acid strength,then transferring the chips and acid to a digester for further treatmentat a different temperature and then transferring the contents of thedigester to a second digester for treatment under different temperatureconditions.

18. The method of digesting wood in a series of steps in differentcontainers which consists in presaturating wood chips in a containerunder theoretically correct conditions of temperature and acid strength,then transferring the chips and acid to a digester for further treatmentat a different temperature and then transferring the contents of thedigester to a second digester for treatment under different temperatureconditions and disintegrating the chips in the process of transferringthe material from the first to the second digesters.

19. In means of the class described, the combination of a presaturatingtank, means for indirectly heating the contents of the tank, a pluralityof digesters, and means for transferring chips and acid from thepresaturator to the first digester and from the first to an adjacentdigester to pro ressively treat the chips under different cont litionsof temperature and acid strength.

20. In means of the class described, the combination of a presaturatingtank, means for indirectly heating the contents of the tank, means forcontinuously mechanically circulating the contents of the tank, aplurality of digesters, and means for transferring chips and acid fromthe presaturator to the first digester and from the first to an adjacentdigester to progressively treat the chips under dliferent conditions oftemperature and acid strength.

21. The method of digesting wood in a series of steps in differentcontainers which consists in presaturating wood chips in a containerunder theoretically correct conditions of temperature and acid strength,then transferring the chips and acid to a second container for furthertreatment at a different temperature and then transferring the contentsof the said second container to a third containenfor treatment underdifferent temperature conditions.

22. The method of digesting wood in a series of steps in differentcontainers which consists in presaturating wood chips in acontainer'under theoretically correct conditions of temperature and acidstrength, then transferring the chips and acid to a second container forfurther treatment at a different temperature and then transferring thecontents of the said second container to a third container for treatmentunder different temperature conditions and disintegrating the chips inthe process of transferring the materlal from one to another of thecontainers.

In testimony whereof I have affixed my signature. PAUL E. HODGDON.

